Contact free power transfer has been around for a long time. A case in point is the transformer which converts high voltage AC power into low voltage AC power and vice versa through a shared inductive magnetic field. The inductive magnetic filed is generated by the primary coil which induces an electromotive force that propels electrons in the secondary conducting coil with the aid of a high magnetic permeability material core. There are no direct electrical conductive contacts between the primary and the secondary coils. A modern example of a transformer is the Braun Oral B toothbrush, which relies on the external casing of the toothbrush base to guide the toothbrush to mate with the internal shape of the charger module, thus allowing the primary coil, the secondary coil, and the high permeability cores to align perfectly so that charging can take place. Other induction charging device prior arts are similarly restrictive in the way the primary unit mates with the secondary unit. For those prior art inventions, the secondary device can only be charged by a dedicated primary device designed for it and charging is only possible if the primary unit is aligned accurately to receive the secondary unit it is designed for.
The contact free power transfer concept as embodied by the transformer inherently requires almost perfect mechanical alignment. The reason for this is that any “magnetic flux leakage” between the primary coil and the secondary coil, namely, the magnetic flux that is generated by the primary coil that is not enveloped by the secondary coil, will be seen by the primary unit as a series leakage inductance. The leakage inductance is directly proportional to both the self inductance of the primary coil and the fractional loss of magnetic coupling between the primary and the secondary. Since the power transfer rate is directly proportional to the primary self inductance as well as to the magnetic coupling coefficient and the frequency of the AC source current, the self inductance of the primary coil must necessarily be high. Hence even a 1% flux leakage will result in leakage impedance that is large enough to practically choke off the primary current. The leakage flux can only be minimized by properly aligning the secondary coil with the primary coil and by using the high permeability core to concentrate and guide the inductive magnetic flux through the volume enclosed by the coils with minimum stray magnetic fields. To further minimize leakage, the air gap between the primary magnetic core and the secondary magnetic core must be kept as small as possible. This explains why an improperly seated inductive toothbrush oftentimes will fail to get charged. The requirement for precise alignment presents a major obstacle to the popularity of such devices.
To surmount the difficulty of transformer-based inductive power transfer, one or more capacitive load may be introduced in series with the primary circuit in order to compensate for the large inductive leakage impedance. This is similar to “power factor correction” for power AC circuits. A switched capacitor bank can be used to adjust the capacitive load to accurately cancel out the inductive load caused by the leakage flux. A primary circuit in which the series capacitive load and series inductive load precisely cancels is called a series resonant circuit. Power transfer that relies on series resonance is appropriately called “resonant inductive power transfer.” Such transfer is no longer subjected to the stringent leakage free coupling requirement of the transformer power transfer method.
One prime example of resonant power transfer is the passive RFID tag. Passive RFID tag has no internal power supply. The minute RF current induced in the antenna of the tag by the incoming RF signal provides the power for the tag. The power is rectified and stored in a capacitor. When the voltage of the capacitor reached a threshold value, the capacitor activates the digital memory chip that is given a unique electronic product code. The RFID code is used to modulate the antenna to reflect the continuous RF signal transmitted by the RFID “interrogator” by varying the reflectivity of the antenna. The interrogator detects the changes in the reflected power and demodulates and decodes the received RFID signal. The passive RFID represents a diametrically opposite example to an AC transformer in that the leakage flux is so large that the coupling coefficient is for all practical purpose zero, i.e., very little flux from the primary interrogator coil is linked to the secondary passive RFID antenna coil. Hence the leakage coefficient is virtually 1. In such cases, there is no need of a switched capacitor bank. A single fixed capacitor tuned to the series resonance of the primary circuit is sufficient. Because the coupling coefficient is so small, very little power actually gets transferred between the primary unit (the interrogator) and the secondary unit (passive RFID tag), hence the energy conversion efficiency is extremely poor. However, the main purpose of RFID technology is not to provide power transfer, but to enable the interrogator to briefly power up the passive tag's power unit with just enough energy to transmit back the product code information. The fact that there is power transfer between the primary unit and the secondary unit is just a means to achieve that goal without having to put an energy storage device inside the tag.
Another example of resonant power transfer is the Wacom's wireless tablet technology. A Wacom pen or mouse can be used with their tablets as input devices without the need of any internal power supply such as batteries or super-capacitors. The tablet acts as a primary unit with a multitude of primary coil arranged in a matrix and emits a low power magnetic field whose intensity decreases rapidly away from the tablet. The pen or mouse comprises an antenna and a transponder unit that receives the incoming RF power, rectifies it, and retransmits a RF signal at a different frequency. The retransmitted signal is detected by two or more primary coils that are close to the antenna of the secondary unit. The received signals are used to triangulate the precise location of the secondary antenna. Wacom's technology can be considered as a variation of the RFID technology in its use of a frequency converting transponder. Because of the closeness of the primary coils to the secondary antenna, very little power is required to transmit a beacon signal. Hence, as in RFID, the energy conversion efficiency does not play a role here, and the contactless power transfer is just a means to achieve the goal of precisely locating the battery-less pointing device.
US 20030210106A1 to Lily Ka Lai Cheng et al. discloses a system and method for transferring power without the need for direct electrical conductive contacts between a primary unit and one or more secondary units. A salient aspect of that patent is that the inductive magnetic field lines generated by the primary unit are substantially parallel to the plane of the surface within the laminar active surface of the primary unit, which minimizes the intensity and size of the magnetic field generated to reduce electromagnetic emission. Another salient aspect of that invention is the convenience of being able to allow secondary devices to be placed anywhere within the active vicinity thereby eliminating the need for plugging-in or placing secondary devices accurately relative to an adaptor or charger. A still another aspect of the prior art is the ability of the primary unit to supply power to a number of secondary different devices with different power requirements simultaneously. These are achieved by a unique design of two large base coils that generate magnetic fields which are substantially parallel to the plane of the laminar base surface and are substantially orthogonal to each other in space and time. The localization of the magnetic field is further aided by the use of high magnetic permeability material. This overcomes one of the main problems associated with inductive charging: the generation of large magnetic field profile. Since the intensity of electromagnetic emission is governed by regulatory limits, any device that is capable of generating a large inductive magnetic field profile runs the risk of violating such limits. In addition, numerous objects can be adversely affected by the presence of a large magnetic field, causing them to either be heated or lose precious stored data.
Despite these advances, this prior art approach suffers from two serious limitations: First, although the magnetic field generated by such approach has a low profile, it does not provide a method to localize the magnetic field around the secondary devices. Thus, if a metallic object is placed on or near the active area of the primary device, an eddy current will be induced, causing the metal to heat up, with a like effect on the primary unit, which sees the metallic object as an equivalent short, which may lead to irreparable damage to the primary unit. Second, the height of the magnetic field profile for the primary unit is directly proportional to the linear dimensions of the primary unit because of the similarity principle. Hence the concept can't easily be scaled up to large primary devices without exceeding regulatory limits. To date, the assignee of that patent, Splashpower limited, has demoed only small, mouse-mat-sized pad that can support up to two small power devices such as PDAs and cell phones simultaneously. Splashpower's devices have also suffered from low energy conversion efficiency.
Another approach to avoid the generation of large magnetic fields is to use an array of coils whereby only a few of them are activated as need arises. This was suggested by a paper published in the Journal of the Magnetics Society of Japan entitled “Coil Shape in a Desk-type Contactless Power Station System” (Nov. 29, 2001). In one embodiment of the concept, multiple position sensors sense the presence of a secondary device placed on the primary unit and relay that information to a central controlling unit. The control unit then sends power to the appropriate coils to energize the secondary coil in a localized fashion. The degree of localization of the magnetic field is determined by the number of primary coils. Since each coil requires a dedicated high frequency switching unit to activate or deactivate, and the central controller unit needs to have an input pin and an output pin for every coil, the complexity increases drastically as the number of primary coils increases.
The prior art solutions invariably use low induction frequencies in the audio frequency range (1 KHz to 50 KHz). The advantages of such solutions include the ready availability of power generation devices in this frequency range as well as the existence of very high magnetic permeability materials which can increase the induced magnetic flux drastically by offering a low reluctance path for the induction magnetic field, thereby increasing the power transfer. The drawbacks of the prior art approaches are: First, the power generation devices are typically bulky because of the need to use large inductors and capacitors at such low frequencies as well as the relatively large current requirement. Second, the high magnetic permeability materials typically have rather large loss tangent, and have nonlinear and hysteretic behaviors at large magnetic flux intensities. Further, the strong reliance of the prior art approaches on the use of high permeability materials in order to reduce the bulkiness of both the primary and the secondary device means that the only major high magnetic reluctance circuit resides within the air gap between the primary and the secondary devices. The consequence is that the power transfer decreases rapidly as a function of the air gap, rendering the prior art technologies no better than the prior art solution based on direct electrical contact in their ability to tolerate vertical separation. Fourth, in order to enable the power transfer through the air gap, the inductive magnetic flux through the air gap must be sufficiently large owing to the relatively low induction frequency. This is because the power transfer is proportional to the product of the induced voltage and the induced current, and the induced voltage is in turns proportional to the rate of change of magnetic flux linked. In other words, the induced voltage is proportional to the product of the driving frequency and the linked magnetic flux.
Within the air gap, the relative magnetic permeability is 1; hence the magnetic field strength within the air gap cannot be reduced by the presence of high permeability material elsewhere. Since the lower the driving frequency, the larger the magnetic field required for a given power transfer rate, this means that low frequency inductive power transfer necessarily entails a severe size/power tradeoff, namely, for a given size of the induction coil, there is a power limit. For audio frequency inductive methods, the power limit for an inductive coil that can fit into a typical cell phone is limited but sufficient to charge a cell phone. The situation gets worse for larger devices such as a notebook computer which often requires 20-40 times more power to charge its battery but lacks space for a coil large enough for that purpose.
The inability of the majority of prior arts to localize the inductive field presents thorny interference and safety issues, as well as energy conversion efficiency issues. The danger that is inherent in having stray magnetic flux that can cause metallic objects in its vicinity to heat up, no matter how slowly is a major safety hazard. An user wearing a ring on his or her finger, or having a metallic implant, is liable to have induced eddy current in and around his/her body. The electromagnetic interference caused by such stray field can also severely hamper the proper operation of a sensitive electronic device. Lastly, the presence of nonlocalized field which does not couple to client devices drastically reduce the energy conversion efficiency since the Q factor of the primary coil is not infinity in practice, hence the portion of the current that is responsible for the generation of such stray flux will always have a resistive component. The larger the unlinked flux, the higher the dissipation loss. Worse, the magnetic flux does not diminish even in the absence of any client devices in the active vicinity; hence the standby power consumption represents a 100% power loss.
Even though each of the aforementioned problems can be solved in theory, the solutions invariably increase system complexity and drive up cost as well as making the system less reliable and less user-friendly. The hazard associated with the presence of metallic objects in the vicinity of the primary device, for example, can be solved through the use of a proximity metal detector. The energy conversion efficiency can theoretically be improved by using conducting materials with very low resistivity such as room temperature superconductors, etc. The EMI interference issues can exemplarily be addressed by either limiting the size of the primary surface, or by switching micro-coils, as was suggested in the aforementioned Japanese paper (Nov. 23, 2001). The no-load power loss can be addressed by automatically turning off the power to the primary coils when no devices have been detected, and is only turned back on when at least one device has been detected. The detection of an eligible device can be performed by embedding a RFID in a device so that a RFID reader in the primary device can read the RFID tag to identify compatible client devices on the primary surface. The absence of any RFID signature will allow the primary unit to go to a standby mode with no electromagnetic emission.
Although a metal detector will make the prior art system safer, it significantly limits how the prior art system can be used. For example, a nearby metallic object will cause the primary device to go to a standby mode immediately, terminating the power transfer; hence the sensitivity of the metal detector must be tuned carefully to ensure that its envelope more or less coincides with that of the active region of the primary device. This way, a distant metallic object won't trigger an unnecessary shutdown. Even then, any small metallic object that accidentally falls on the primary surface will have to be removed, which is clearly a big annoyance. Worst, if an object such as steel wool whose metallic content is insufficient to trigger the metal detection happens to be on the primary surface, it may still be able to couple to the inductive magnetic field to generate a small Eddy current which causes it to slowly get hot enough to create a fire hazard. Similarly, if the sensitivity and range of the RFID reader is not properly tuned, it may either be unable to detect an eligible client device and therefore fails to turn on the primary coils, or there may be a client device that is too far to be charged by the primary device and yet close enough to be identified by the RFID reader to prevent the primary device from going to the standby mode.